1. Field of the Invention
The invention relates to a mute circuit, and in particular to a mute circuit capable of eliminating audible noise in audio system when the audio system is powered up and powered down.
2. Description of the Related Art
Generally, purchasers of high quality home and portable audio systems expect improved audio performance, as well as more options for controlling playback from the given recording media. One of the most important performance criteria is the elimination of clicks, pops, noise and other artifacts audible to the user. Not only are these audible artifacts distracting, but can also damage the system speakers or headset. This is especially true with transient artifacts, such as clicks and pops, which may spike the output signal driving the speakers or headset to a relatively high level.
In a typical audio system, the loads (e.g., the speakers or headset) are AC coupled to an audio integrated circuit sourcing electrical analog audio signals through a coupling capacitor. The normal quiescent output node voltage is roughly ½ of the power supply voltage. However, before power is supplied to the circuit, the output node voltage at the coupling capacitors is zero volts. Subsequently, when the circuit is powered up, the output node voltage transitions from zero volts to the quiescent voltage, thereby charging the coupling capacitors. This voltage transition can generate an audible sound in the speakers or headset, typically called a “pop.” A “pop” can also occur when the system powers down and when the output nodes transition from quiescent voltage to zero volts.
Thus, improved circuits and methods for eliminating audible noise are needed.